The present invention relates to gas detection systems based on measurement of absorption of electromagnetic radiation by the gas of interest, and particularly to such systems which identify the type of gas present and which measure the concentration of the gas of interest in terms of percentage of a specific level of such gas in ambient air. Such specific level may be the lower explosive limit level of a combustible gas, or the maximum safe level for eight hour exposure by humans to a toxic gas.
In the following description the term xe2x80x9copticalxe2x80x9d will be understood to signify relating to electromagnetic radiation anywhere in the spectrum between the long microwave frequencies and the ultraviolet radiation of less than 100 nm. xe2x80x9cLightxe2x80x9d will be understood as any form electromagnetic flow of energy.
All gases which are the subjects of optical detection are known to exhibit attenuation of radiation in portion of the electromagnetic spectrum. Optical gas detection systems for such gases generally comprise one or more source(s) of electromagnetic radiation which direct their output through a gas to be identified to one or more light detectors. The detectors each respond to a small portion of the electromagnetic spectrum and electrical output levels from the detectors can be used to measure light-attenuation to identify the presence of a particular gas. Although, some success has been achieved in creating such optical gas detectors, they tend to be incapable of distinguishing between similar gases of the same class of gases, such as distinguishing between methane and butane, both of which are hydrocarbon gases. In many cases it is desirable to know not only that gases are present but whether the concentration of such gases are near a limit which might cause a problem. For example, when a hydrocarbon gas may be present it would be desirable to know if the gas concentration is at or near the level at which an explosion might occur. This limit is referred to herein as the lower explosive limit level or LEL level.
With known detectors it is possible to detect the presence of hydrocarbon gases by measuring the attenuation of infrared (IR) light in a detector as described above. Without knowing a priori which hydrocarbon gas or vapor is present, it is not possible to indicate the concentration of gas present or whether the detected IR attenuation may indicate a possibly explosive mixture or not.
The industrial environment also creates problems relating to continuous sampling of changing sampled air. That is, how can suspect air be substantially continuously evaluated by a detector in an industrial environment and still be expected to achieve the necessary gas discrimination and accuracy in lower explosive limit level detection.
The above problems are solved and an advance is achieved in accordance with the present invention. As disclosed herein, an apparatus for detecting and measuring concentration of gases comprises a chamber containing an air/gas mixture to be tested and one or more light sources for directing. light through the air/gas mixture to one or more light detectors. The light source(s) and detector(s) are used as described hereinafter to define a plurality of detection channels each for responding to a specific band of the light spectrum. The apparatus may also include a reference channel which is similar to the detection channels but responds to a light band which normally does not exhibit attenuation for the gases expected. Output signals from the detectors are sampled by a controller and used to measure the absorption by the air/gas mixture of light in the bands individually associated with the detection channels. The apparatus is preprogrammed with a signature table for each gas expected. Each signature table stores values representing expected output signals in the detection channels for a plurality of concentrations of the gas represented by the table. When three detection channels are used by the apparatus, each gas signature table will include three sets of entries, one for each detection channel, corresponding to each of a plurality of concentrations of the gas represented by the table.
The controller regularly samples the output signals in the detection and reference channels and normalizes the signals to render them independent of variations in hardware characteristics due to production fluctuations, and to render them independent to variations due to aging of the equipment and compensation of temperature effects. A first normalization is obtained by dividing each detection and reference channel output signal by the value of the same channel when the gas detection apparatus was presented with an atmosphere devoid of any of the gases to be detected, resulting in a normalized detector channel signal. A second normalization is obtained by dividing each normalized detection channel signal by the normalized reference channel signal.
After normalization, the controller compares the detection channel signal values to the various expected signal values of the signature tables. In one embodiment, such comparison is done by selecting a possible candidate gas contained in the signature tables, reading its signature table and computing from the signature table the gas concentration for each of the signal values using standard interpolation. If the multiple gas concentrations thus calculated do not match each other the selected candidate gas is different from the gas present in the test, and a next gas in the signature tables is selected to repeat the calculation. If the multiple gas concentrations thus calculated do match each other the selected candidate gas is the gas present in the test, and the concentration of the gas is the concentration calculated from the signature table. As long as a substantial match is not detected in a candidate signature table, other gas signature tables are checked one at a time. If none of the gas signature tables furnish a substantial match a default gas signature table is used to force a gas concentration reading. Advantageously, the signature tables may be grouped in accordance with a common characteristic so that some tables may be excluded based on the sampled detection channel output signals. For example, when three detection channels numbered 1, 2 and 3 are used, the detection channel signals are compared with one another and arranged in ascending order of detected absorption. Beforehand, the signature tables are also grouped according to ascending order of expected absorption, and only signature tables having the same characteristics as the detection channel output signals are compared with the detection channel output signals. In the example using detection channels 1, 2 and 3, their output signals, arranged by ascending absorption may be 3-1-2. These detection channel output signals are compared only to signature tables having expected output signals arranged in the 3-1-2 sequence.
In an embodiment of the gas detection and measurement system a weighted average of the concentration identified from the signature tables is computed. The weighted average computation is particularly valuable in measurements which are very close to the detection limits of one of the detection channels and in situations where a significant change in gas concentration does not cause a significant change in measured infrared absorption. The last named situation may occur, for example, at very low concentrations of gas and at high concentrations. In the weighted average computation, a concentration level is identified for the signal output of each of a plurality of detection channels. The identified concentration levels are each multiplied by a weighting factor representing the change of absorption per change in gas concentration, and an average is computed by dividing the sum of the weighted concentration by the total of the weighting factors. This method reduces the influence on the final measured concentration of infrared absorption readings which may not be representative.
The apparatus may be constructed as a controller unit including a processor, display and input device and a changeable measuring head. The measuring head may include the light sources and detectors to define the detection channels and a persistent memory such as EPROM to store signature tables and normalizing values for gases expected to be detected by that particular measuring head. With this construction the system can be reconfigured for testing for different sets of gases by simply changing the measuring head.